Here are a few techniques the data team uses to quickly trace buildings for OpenStreetMap. First, we use JOSM, the OpenStreetMap desktop data editor. We then install a couple custom plugins specifically for tracing buildings, like building_tools which lets us trace rectangular buildings in 2-3 mouse clicks and utilsplugin2 (see also) which is a collection of super useful tools for JOSM.
Here is a look at how the plugins work. Choose an area for building tracing and download data into JOSM. Check the imagery offset and adjust if needed.
There are two main methods for drawing buildings: extrusion and area intersections.
Select Draw buildings (or just press shortcut - B), and draw the basic rectangular outline of the building.
“Extrude” part of the building to add extra details. Select the extrusion tool X. Add an extra node with mouse double-click, then pull out needed segment of building.
Using area intersections
To create more complex buildings, draw a first box over the building, and then select 2 nodes of that box. Draw additional boxes to cover the building footprint; they will be parallel to the first box.
Then select all traced parts of the building. First select one of them and then press I (to select intersected ways one by one); or ⌘+* (to select all intersecting ways).
Now combine all selected parts of the building - press Shift+J to Joining overlapping Areas
We just traced a complex building outline in 5-8 seconds!